SiC SUBSTRATE WITH SiC EPITAXIAL FILM

ABSTRACT

A method of forming an epitaxial SiC film on SiC substrates in a warm wall CVD system, wherein the susceptor is actively heated and the ceiling and sidewall are not actively heated, but are allowed to be indirectly heated by the susceptor. The method includes a first process of reaction cell preparation and a second process of epitaxial film growth. The epitaxial growth is performed by flowing parallel to the surface of the wafers a gas mixture of hydrogen, silicon and carbon gases, at total gas velocity in a range 120 to 250 cm/sec.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of, and priority to, U.S.Provisional Patent Application No. 61/798,819, filed on Mar. 15, 2013,entitled “SiC SUBSTRATE WITH SiC EPITAXIAL FILM,” the entire disclosureof which is hereby incorporated herein by reference.

BACKGROUND

1. Field

This invention relates to fabrication of silicon carbide substrates and,more particularly, to silicon carbide substrates having epitaxial filmgrown thereupon.

2. Related Art

Silicon carbide, SiC, is a crystalline semiconductor material,recognized by those familiar with materials science, electronics andphysics as being advantageous for wide band gap properties and also forextreme hardness, high thermal conductivity and chemical inertproperties. These properties make SiC a very attractive semiconductorfor fabrication of power semiconductor devices, enabling power densityand performance enhancement over devices made from more common materialslike silicon.

The most common forms of SiC consist of cubic or hexagonal arrangementsof atoms. The stacking of Si and C layers can take on many forms, knownas polytypes. The type of silicon carbide crystal is denoted by a numberdenoting the number of repeat units in the stacking sequence followed bya letter representing the crystalline format. For example the 3C-SiCpolytype refers to a repeat unit of 3 and a cubic (C) lattice, while a4H-SiC polytype refers to repeat unit of 4 and a hexagonal (H) lattice.

The different silicon carbide polytypes have some variations inmaterials properties, most notably electrical properties. The 4H-SiCpolytype has the relatively larger bandgap while the 3C-SiC has asmaller bandgap, with the bandgaps for most other polytypes falling inbetween. For high performance power device applications when the bandgapis larger, the material is more capable, in theory, to offer relativelyhigher high power and thermal conductivity performance.

SiC crystals do not occur in nature and as such must be synthesized.Growth of SiC crystals can be executed by sublimation/physical vaportransport or chemical vapor deposition.

Once SiC crystals are produced, each crystal must be cut and fabricatedinto wafers using planar fabrication methods to fabricate semiconductordevices. As many semiconductor crystals (e.g., silicon, galliumarsenide) have been successfully developed and commercialized into waferproducts, the methods to fabricate wafers from bulk crystals are known.A review of the common approaches to, and requirements for waferfabrication and standard methods of characterization can be found inWolf and Tauber, Silicon Processing for the VLSI Era, Vol. 1—ProcessTechnology, Chapter 1 (Lattice Press—1986). Due to its hardness,fabrication of SiC into wafer substrates presents unique challengescompared to processing other common semiconductor crystals like siliconor gallium arsenide. Modifications must be made to the machines, and thechoices of effective abrasives are beyond commonly used materials. Themodifications made to common wafer fabrication techniques in order toaccommodate SiC are often kept as proprietary information. However, ithas been reported that substantial subsurface damage is observable onmirror polished SiC wafers, and this can be reduced or removed by usingchemical enhanced mechanical polishing methods similar to that used inthe silicon industry (Zhou, L., et al., Chemomechanical Polishing ofSilicon Carbide, J. Electrochem. Soc., Vol. 144, no. 6, June 1997, pp.L161-L163).

In order to build semiconductor devices on SiC wafers, additionalcrystalline SiC films must be deposited on the wafers, so as to createthe device active regions with the required conductivity value andconductor type. This is typically done using chemical vapor deposition(CVD) methods. Techniques for growth of SiC by CVD epitaxy have beenpublished from groups in Russia, Japan and the United States since the1970's. The most common chemistry for growth of SiC by CVD is a mixtureof a silicon containing source gas (e.g., monosilanes or chlorosilanes)and a carbon containing source gas (e.g., a hydrocarbon gas). A keyelement to growth of low defect epitaxial layers is that the substratesurface is tilted away from the crystal axis of symmetry, to allow thechemical atoms to attach to the surface in the stacking orderestablished by the substrate crystal. When the tilt is not adequate, theCVD process will produce three dimensional defects on the surface, andsuch defects will result non-operational semiconductor devices. Surfaceimperfections, such as cracks, subsurface damage, pits, particles,scratches or contamination will interrupt the replication of the wafer'scrystal structure by the CVD process (see, for example, Powell andLarkin, Phys. Stat. Sol. (b) 202, 529 (1997)). Therefore, it isimportant that the polishing and cleaning processes used to fabricatethe wafer minimize surface imperfections. In the presence of thesesurface imperfections several defects can be generated in the epitaxialfilms, including basal plane dislocations and cubic SiC inclusions (seefor example, Powell, et. al. Transactions Third InternationalHigh-Temperature Electronics Conference, Volume 1, pp. 11-3 -11-8,Sandia National Laboratories, Albuquerque, N. Mex. USA, 9-14 June 1996).

The methods of SiC epitaxy have been reviewed by G. Wagner, D. Schulz,and D. Siche in Progress in Crystal Growth and Characterization ofMaterials, 47 (2003) p. 139-165. Wagner discusses that SiC epitaxy canachieve favorable results if performed in a hot wall reactor, where allthe surfaces of the reaction cell that are exposed to gases, includingthe susceptor that holds the SiC substrate, are actively heated. This isin contrast to a cold wall reactor where only the susceptor supportingthe SiC substrate is actively heated, while the other surfaces areactively cooled or designed not to heat. Today there is also a so calledwarm wall CVD system, which is an intermediate of the hot and cold walldesign, where the susceptor of the reaction cell supporting the SiCsubstrate is actively heated, and top and side surfaces of the celladjacent to this heated surface are allowed to be indirectly heated.Warm wall CVD systems capable of depositing SiC epitaxy on severalwafers simultaneously have emerged for commercial applications. Suchsystems have been described by Burk, Jr. (U.S. Pat. No. 5,954,881),Jurgensen, et. al., (WO 2002018670), and Hecht, et. al., (MaterialsScience Forum Vols. 645-648 (2010) pp. 89-94).

Defects in SiC are known to limit or destroy operation of semiconductordevices formed over the defects. Neudeck and Powell reported that hollowcore screw dislocations (micropipes) severely limited voltage blockingperformance in SiC diodes (P. G. Neudeck and J. A. Powell, IEEE ElectronDevice Letters, vol. 15, no. 2, pp. 63-65, (1994)). Neudeck reviewed theimpact of crystal (wafer) and epitaxy originated defects on powerdevices in 1994, highlighting limitations of power device function dueto screw dislocations and morphological epitaxy defects (Neudeck, Mat.Sci. Forum, Vols. 338-342, pp. 1161-1166 (2000)). Hull reported shift tolower values in the distribution of high voltage diode reverse biasleakage current when the diodes were fabricated on substrates havinglower screw dislocation density (Hull, et. al., Mat. Sci. forum, Vol.600-603, p. 931-934 (2009)). Lendenmann reported forward voltagedegradation in bipolar diodes was linked to basal plane dislocations inthe epilayer that originate from basal plane dislocations in thesubstrate (Lendenmann et. al., Mat. Sci. Forum, Vols. 338-342, pp.1161-1166 (2000)).

3. Problem Statement

Advances in SiC substrate and epitaxy are required in order to reducethe concentration of defects that impact device operation andfabrication yields. Currently, defects formed on the surface of thesubstrate during SiC CVD epitaxy are the most influential defectimpacting operation and yields of semiconductor devices on SiCsubstrates. In particular, SiC power devices which are required tohandle large current (>50 A) with low on resistance are made usingrelatively large die sizes, greater than 7 mm per side. To achieve goodmanufacturing yield of these devices, methods to further reduce CVDepitaxy originated defects need to be developed. Solutions of theseproblems must also be capable of producing repeatable and consistentdeposition of films that are smooth, uniform in thickness and electricalproperties so that these parameters are still consistent with highdevice fabrication yields.

In multi-wafer, warm wall SiC CVD systems, reactant gas is introduced toa graphite reaction zone in the center of the system, the gas flow fansout in the radial direction and parallel to the substrate surface, andis finally evacuated at the periphery of the chamber. The floor of thereaction zone, or susceptor, contains the substrates and is activelyheated, making it the hottest point in the reaction zone. Heating of thesusceptor may be done using RF induction techniques or by resistiveheaters. The adjacent surfaces are indirectly heated by the susceptor atthe bottom of the chamber, and are at lower temperatures than thetemperature target of the susceptor. Due to the control temperaturesrequired for SiC CVD epitaxy, the reaction cell is constructed fromgraphite. Prior to its use, the parts of the reaction zone are oftencoated with pyrocarbon or tantalum carbide films which act as barriersto the out diffusion of impurities from the graphite. During CVD,ancillary deposits of SiC rapidly grow on the adjacent surfaces at arate faster than the susceptor/substrate surface. Often, a mask, such asplates of polycrystalline SiC, can be laid over the uncovered regions ofthe susceptor to mask or protect areas of the susceptor from ancillarydeposits. When these ancillary deposits reach a critical thickness, theywill shed particles onto the substrates, resulting in defects in theepitaxial film that will impair operation of semiconductor devices. Inaddition, the formation of ancillary deposits consumes process gasreactant which can lead to run-to-run variations in film properties,film surface morphology, and particularly electrical properties.

SUMMARY

The following summary is included in order to provide a basicunderstanding of some aspects and features of the invention. Thissummary is not an extensive overview of the invention and as such it isnot intended to particularly identify key or critical elements of theinvention or to delineate the scope of the invention. Its sole purposeis to present some concepts of the invention in a simplified form as aprelude to the more detailed description that is presented below.

Various disclosed embodiments provide control of CVD SiC epitaxyoriginated defects, surface roughness, epitaxy film thickness, epitaxyfilm doping and run-to-run consistency, using a multi-wafer CVD system.

According to one aspect, a method is described providing a strategy ofgas flow and temperature control to ensure optimal distribution ofgaseous chemical reactants in the reaction zone.

According to another embodiment, a method is described providing apretreatment of an unused reaction cell to coat it with a layer of SiCin such a way that the future ancillary deposits have good adhesion tothe original reaction cell surfaces and avoid growth morphology that canresult in separation of the deposits from the walls and particlesfalling on the substrates. This process would be repeated when a new orreconditioned reaction cell is placed into the CVD system.

In one aspect, provided herein is a method of manufacturing a 4H-SiCepiwafer comprising an epitaxial SiC film on a single-crystal 4H-SiCsubstrate, the method comprising: loading the single-crystal 4H-SiCsubstrate onto a susceptor in a reaction cell of a warm wall CVD system;heating the system by controlling a temperature of the susceptor in thereaction cell to a range from 1500° C. to 1620° C.; and executing amanufacturing run to produce the 4H-SiC epiwafer, the manufacturing runcomprising supplying a gas flow parallel to a surface of thesingle-crystal 4H-SiC substrate, such that a total gas velocity is in arange from 120 to 250 cm/sec and controlling a pressure inside thereaction cell to a range from 100 to 150 mbar, wherein the gas flowcomprises a mixture of hydrogen gas, silicon gas, and carbon gas, inorder to produce the epitaxial SiC film on the single-crystal 4H-SiCsubstrate.

In one embodiment of this aspect, the method further comprisesestablishing a temperature gradient from a surface of the epiwafer to aceiling of the reaction cell in a range from 25° C./cm to 80° C./cm.

In another embodiment of this aspect, the method further comprises:measuring performance metrics of the 4H-SiC epiwafer; and removing theused reaction cell when the measured performance metrics of the 4H-SiCepiwafer fall below acceptable threshold limits.

In another embodiment of this aspect, the step of loading thesingle-crystal 4H-SiC substrate onto the susceptor in the reaction cellof the warm wall CVD system is preceded by pre-treating the reactioncell, the pre-treating comprising: loading a sacrificial substrate ontothe susceptor in the reaction cell; sealing and evacuating the reactioncell; purging the reaction cell using inert and hydrogen gases; bakingthe reaction cell at a temperature in the range 1400° C. to 1700° C.while flowing hydrogen gas mixed with 1% to 10% hydrocarbon gas;performing a CVD deposition process so as to deposit an SiC film on thesidewall and ceiling of the reaction cell; and removing the sacrificialsubstrate from the reaction cell.

In another embodiment of this aspect, the step of executing amanufacturing run to produce the 4H-SiC epiwafer comprises: placing thesingle-crystal 4H-SiC substrate on the susceptor in the reaction cell ofthe warm wall CVD system; evacuating the reaction cell and then purgingthe reaction cell with argon; terminating the argon flow and initiatinghydrogen gas flow into the reaction cell; establishing the temperaturegradient from the surface of the epiwafer to the ceiling in the rangefrom 25° C./cm to 80° C./cm; flowing parallel to the surface of theepiwafer a gas mixture of hydrogen, silicon and carbon gases, at a totalgas velocity of 120 to 250 cm/sec; maintaining the process conditions toachieve a total deposit of from 3 to 120 μm of film on thesingle-crystal 4H-SiC substrate in order to produce the epitaxial SiCfilm on the single-crystal 4H-SiC substrate; cooling the system to atemperature less than 300° C.; and removing the single-crystal 4H-SiCsubstrate.

In another embodiment of this aspect, the method further comprisestesting the performance of the single-crystal 4H-SiC substrate.

In another embodiment of this aspect, in the step of maintaining theprocess conditions to achieve the total deposit of from 3 to 120 μm offilm on the single-crystal 4H-SiC substrate in order to produce theepitaxial SiC film on the single-crystal 4H-SiC substrate, the gas flowfurther comprises doping gas.

In another embodiment of this aspect, the method further comprisesflowing an etching gas into the reaction cell to etch the single-crystal4H-SiC substrate prior to the step of maintaining the process conditionsto achieve the total deposit of from 3 to 120 μm of film on thesingle-crystal 4H-SiC substrate in order to produce the epitaxial SiCfilm on the single-crystal 4H-SiC substrate.

In another embodiment of this aspect, the etching gas comprises ahalogen gas and hydrogen.

In another embodiment of this aspect, the single-crystal 4H-SiCsubstrate comprises a polished 4H-SiC wafer with a diameter ranging from100 to 200 mm and having a nitrogen concentration of at least1×10¹⁸/cm³.

In another embodiment of this aspect, the epitaxial SiC film isdeposited on an exposed silicon surface of the single-crystal 4H-SiCsubstrate.

In another embodiment of this aspect, the epitaxial SiC film isdeposited on an exposed carbon surface of the single-crystal 4H-SiCsubstrate.

In another embodiment of this aspect, in the step of loading thesingle-crystal 4H-SiC substrate onto a susceptor in a reaction cell of awarm wall CVD system, a plurality of five to twelve single-crystal4H-SiC substrates are placed in the reaction cell.

In another embodiment of this aspect, when in the step of loading thesingle-crystal 4H-SiC substrate onto a susceptor in a reaction cell of awarm wall CVD system, a plurality of single-crystal 4H-SiC substratesare placed on the susceptor in the reaction cell of the warm wall CVDsystem.

In another embodiment of this aspect, the reaction cell comprises agraphite reaction cell, and further comprises coating the reactioncell's graphite components with pyrocarbon or tantalum carbide filmsprior to assembling the cell for use in CVD epitaxy.

In another embodiment of this aspect, a single-crystal 4H-SiC substratewith epitaxial SiC film is produced by the method, and a within wafertotal thickness variation of the SiC epitaxial film is from 2 to 12%,inclusive; a within wafer dopant concentration of the epitaxial SiC filmis from 5 to 40%, inclusive; a top surface of the epitaxial SiC film hasan RMS roughness value of 0.2 to 1.2 nm; inclusive; and a density ofsurface defects on the epitaxial SiC film is from 0.25 to 2.0/cm²,inclusive.

In another aspect, provided herein is a method of forming a 4H-SiCepiwafer comprising an epitaxial SiC film on a single-crystal 4H-SiCsubstrate positioned on a susceptor in a warm wall CVD system, the warmwall CVD system having a reaction cell comprising the susceptorpositioned at a bottom, a sidewall, and a ceiling, wherein the susceptoris actively heated, and the ceiling and sidewall are not activelyheated, but are allowed to be indirectly heated by the susceptor, themethod comprising: inserting an unused reaction cell assembly into a CVDepitaxy system; a first process for treatment of the reaction cellpreparation; and a second process for epitaxial film growth; wherein:the first process for treatment of the reaction cell preparationcomprises the steps: loading the susceptor with a sacrificial substrate;sealing and evacuating the reaction cell; purging the reaction cellusing argon and hydrogen gases; baking the reaction cell in a mixture ofhydrogen and hydrocarbon gas; and performing a CVD deposition process soas to deposit an SiC film on the sidewall and ceiling of the reactioncell; and the second process for epitaxial film growth comprises thesteps: allowing the reaction cell to cool; placing the single-crystal4H-SiC substrate on the susceptor; evacuating and then purging thereaction cell with argon gas; heating the susceptor to a temperature of1200° C. to 1400° C., inclusive; terminating the argon flow andinitiating hydrogen gas flow into the reaction cell; flowing parallel tothe surface of the wafers a gas mixture of hydrogen, silicon and carbongases, at a total gas velocity of 120 to 250 cm/sec; maintaining atemperature of the susceptor at 1500° C. to 1620° C.; and controlling apressure inside the reaction cell at from 100 to 150 mbar, inclusive, toachieve a total deposit of 3 to 120 μm of film on the substrate.

In one embodiment of this aspect, the step of baking the unused reactioncell comprises heating the reaction cell to a temperature of from 1400°C. to 1700° C. and maintaining the temperature for from 4 to 24 hours.

In another embodiment of this aspect, a ratio of volumetric flow ofcarbon to silicon gases is less than 1, but greater than 0.05.

In another embodiment of this aspect, the temperature of the susceptor,the total gas velocity, and a throttle setting of the reaction cell arecontrolled so as to maintain a temperature gradient from the substrateto the ceiling in a range from 25° C./cm to 80° C./cm.

In another embodiment of this aspect, the total gas velocity ismaintained at 120 to 160 cm/sec, inclusive.

In another embodiment of this aspect, the total gas velocity ismaintained at 175 to 250 cm/sec, inclusive.

In another embodiment of this aspect, the step of maintaining atemperature of the susceptor at 1500° C. to 1620° C. of the secondprocess further comprises establishing a temperature gradient from the4H-SiC substrate to the ceiling of 25° C./cm to 80° C./cm.

In another embodiment of this aspect, a plurality of single-crystal4H-SiC substrates are placed in the reaction cell.

In yet another aspect, provided herein is a 4H-SiC substrate with anepitaxial SiC film deposited on one surface thereof; wherein, a withinwafer total epitaxy film thickness varies from 2 to 12%, inclusive; awithin wafer dopant concentration of each epitaxy layer varies from 5 to40%, inclusive; a top surface of the epitaxy film has an RMS roughnessvalue in a range from 0.2 to 1.2 nm, inclusive; and a density of surfacedefects on the film is from 0.25 to 2.0/cm², inclusive.

In still another aspect, provided herein is a method of forming anepitaxial SiC film on a single-crystal 4H-SiC substrate in a warm wallCVD system, the warm wall CVD system having a reaction cell comprising asusceptor positioned at a bottom, a sidewall, and a ceiling, the methodcomprising: positioning the single-crystal 4H-SiC substrate on thesusceptor; evacuating the reaction cell and then purging the reactioncell with argon; actively heating the susceptor to a temperature of1200° C. to 1400° C. and allowing the ceiling and sidewall to beindirectly heated by the susceptor, thereby creating a decreasingtemperature gradient from the susceptor to the ceiling; terminating theargon flow and initiating hydrogen gas flow into the reaction cell;establishing the temperature gradient from the wafer surface to theceiling in a range from 25° C./cm to 80° C./cm via heating the system bycontrolling the temperature of the susceptor to a range from 1500° C. to1620° C.; supplying a gas flow parallel to the surface of the substrate,such that total gas velocity is in a range from 120 to 250 cm/sec andcontrolling the pressure inside the reaction cell to a range from 100 to150 mbar, and wherein the gas flow comprises a mixture of hydrogen gas,silicon gas, and carbon gas; maintaining the process conditions toachieve a total deposit of from 3 to 120 μm of film on the substrate;and cooling the system to an ambient temperature.

In one embodiment of this aspect, the reaction cell comprises a graphitereaction cell, and further comprises coating the reaction cell'sgraphite components with pyrocarbon or tantalum carbide films prior toassembling the cell for use in CVD epitaxy.

In another embodiment of this aspect, the reaction cell comprises agraphite reaction cell, and further comprises coating the reactioncell's interior with an SiC film prior to the step of positioning thesingle-crystal 4H-SiC substrate on the susceptor.

In another embodiment of this aspect, the reaction cell's interior iscoated with the SiC layer by the steps: evacuating the reaction cell andthen purging the reaction cell; flowing a gas mixture of Si and Cprecursors where a gas volumetric flow ratio of carbon to siliconprecursors, taken as a ratio of (number of C atoms in precursormolecule)×(carbon volume flow)/(silicon volume flow), is less than onebut greater than 0.05; and heating the reaction cell to form SiCdeposits on the reaction cell's interior.

In another embodiment of this aspect, the gas mixture further compriseshydrogen.

In another embodiment of this aspect, the method further comprises astep of heating the reaction cell to a temperature from 1400° C. to1700° C. and baking the reaction cell for from 4 to 24 hours prior tothe step of evacuating the reaction cell and then purging the reactioncell with argon.

In another embodiment of this aspect, the step of actively heating thesusceptor to the temperature of 1200° C. to 1400° C. and allowing theceiling and sidewall to be indirectly heated by the susceptor, therebycreating the decreasing temperature gradient from the susceptor to theceiling is performed until the SiC deposits reach a thickness thatequates to a thickness of 5 to 10 μm as deposited on the SiC wafers.

In another embodiment of this aspect, at the step of supplying the gasflow parallel to the surface of the substrate, such that total gasvelocity is in the range from 120 to 250 cm/sec and controlling thepressure inside the reaction cell to the range from 100 to 150 mbar, andwherein the gas flow comprises the mixture of hydrogen gas, silicon gas,and carbon gas, a flow of donor or acceptor dopant gas is supplied.

In another embodiment of this aspect, a plurality of single-crystal4H-SiC substrates are placed in the reaction cell.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, exemplify the embodiments of the presentinvention and, together with the description, serve to explain andillustrate principles of the invention. The drawings are intended toillustrate major features of the exemplary embodiments in a diagrammaticmanner. The drawings are not intended to depict every feature of actualembodiments nor relative dimensions of the depicted elements, and arenot drawn to scale.

FIG. 1 is a flow chart illustrating a pre-treat process according to oneembodiment;

FIG. 2 is a flow chart illustrating an embodiment for CVD epitaxialgrowth of film on the SiC substrates;

FIG. 3 is a flow chart illustrating an embodiment of a method of formingan epitaxial SiC film on a plurality of SiC substrates;

FIG. 4 is a flow chart illustrating an embodiment of a first process fortreatment of the reaction cell preparation;

FIG. 5. Is a flow chart illustrating an embodiment of at least onesecond process for epitaxial film growth; and

FIG. 6 depicts an embodiment of a single-crystal hexagonal SiC substratewith at least one epitaxial layer formed according to the methodillustrated in FIGS. 3 to 5.

DETAILED DESCRIPTION

It should be understood that this invention is not limited to theparticular methodology, protocols, etc., described herein and as suchmay vary. The terminology used herein is for the purpose of describingparticular embodiments only, and is not intended to limit the scope ofthe present invention, which is defined solely by the claims.

As used herein and in the claims, the singular forms include the pluralreference and vice versa unless the context clearly indicates otherwise.Other than in the operating examples, or where otherwise indicated, allnumbers expressing quantities used herein should be understood asmodified in all instances by the term “about.”

All publications identified are expressly incorporated herein byreference for the purpose of describing and disclosing, for example, themethodologies described in such publications that might be used inconnection with the present invention. These publications are providedsolely for their disclosure prior to the filing date of the presentapplication. Nothing in this regard should be construed as an admissionthat the inventors are not entitled to antedate such disclosure byvirtue of prior invention or for any other reason. All statements as tothe date or representation as to the contents of these documents isbased on the information available to the applicants and does notconstitute any admission as to the correctness of the dates or contentsof these documents.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as those commonly understood to one of ordinaryskill in the art to which this invention pertains. Although any knownmethods, devices, and materials may be used in the practice or testingof the invention, the methods, devices, and materials in this regard aredescribed herein.

The following examples illustrate some embodiments and aspects of theinvention. It will be apparent to those skilled in the relevant art thatvarious modifications, additions, substitutions, and the like can beperformed without altering the spirit or scope of the invention, andsuch modifications and variations are encompassed within the scope ofthe invention as defined in the claims which follow. The followingexamples do not in any way limit the invention.

The following description provides examples for performing SiC epitaxialgrowth, while minimizing defects. The methods are implemented in a warmwall CVD reactor.

The optimal process parameters (temperature, pressure, gas flow) foroperation of the CVD growth process were first determined. Using theseprocess parameters, the “characteristic” parameters for the CVD processand reactor configuration were determined using finite element models.From this modeling work, it was determined that the importantcharacteristic parameters for warm wall SiC growth are gas velocity andthe temperature gradient between the wafer growth surface and the top ofthe reaction zone.

FIG. 1 is a flow chart illustrating a pre-epitaxy, conditioningtreatment process 100 for a new or reconditioned reaction cell assemblyaccording to one embodiment. The conditioning treatment process is atwo-step method which consists of heating the reaction cell and thendepositing an SiC coating on the reaction cell surfaces. The method wasimplemented in a multi-wafer, warm wall CVD system with a consumablereaction cell assembled from graphite parts which are coated with CVDcoatings of either pyrocarbon or tantalum carbide. Plates or masks ofpolycrystalline SiC may be used to cover open areas of the susceptor and“sacrificial” wafers are placed in the wafer pockets, such wafers arenot products of the process and only used to protect the pockets fromdeposition. The regions of the susceptor that do not hold wafers are, insome embodiments, covered by masks or plates of polycrystalline SiC. Inthis manner, the susceptor is totally protected and the ancillarydeposits are formed on the plates and the sacrificial wafers and not onthe susceptor surface. These plates can be periodically removed and theSiC deposits machined away by grinding. Then, the plates can be reused.This process can prolong the life of the susceptor. The masks/plates areused 100% of the time the machine is run.

Following the process of FIG. 1, first the system is sealed, evacuated(step 101) and purged (step 105) with an inert gas such as argon, and/ora gas mixture containing an inert gas and hydrogen to eliminateatmospheric gas impurities. Next, in step 110 a gas flow of H₂, andoptionally an additional flow of hydrocarbon gas, is established and thepressure controlled using a throttle valve to a value in a range of from100 to 900 mbar, inclusive. The fraction of hydrocarbon gas is typicallyin the range 0 to 10% by volume. The system is then heated to atemperature from 1400° C. to 1700° C. and baked for from 4 to 24 hours(step 115). Once the system has been baked, a base coating of SiC isthen applied to the reaction cell surfaces using a CVD process withsilicon and carbon precursor gases, and with sacrificial substratesloaded in the susceptor pockets to prevent deposition in the pocket.This chamber-wall coating facilitates good adhesion of subsequentdeposits on the surfaces not covered by the wafers (step 120).

Gaseous precursors for Si and C are added to the H₂ gas flow to form aSiC film on the reaction cell's sidewall and ceiling. The flow of Si andC precursors are set to conditions where the gas volumetric flow ratioof carbon to silicon precursors, taken as the ratio of (number of Catoms in precursor molecule)×(carbon volume flow)/(silicon volume flow),is less than one but greater than 0.05. The pressure is set into a rangeof 100 to 200 mbar. The process conditions are held in the reaction zonesuch that a film coats the inside surfaces of the reaction cell, thisfilm equates to a thickness of 5 to 10 μm as deposited on the SiCwafers. This level of wafer deposit is sufficient to form a SiC coatingwith good adhesion to the surfaces in the reaction zone and thepretreatment process is completed by terminating the flow of Si and Cprecursors and cooling the system to ambient conditions under H₂ flow.The sacrificial substrates are removed from the system, and the systemis ready to be used to produce SiC epitaxy wafer products. Typically thesystem is loaded with polished hexagonal, single-crystal SiC substratesof the 4H-SiC polytype. As the epitaxy process is repeated, coatingsform on the walls of the cell, and these coatings will slowlydeteriorate the cell material, change the temperatures in the reactioncell and also flake off and fall onto the SiC substrates. When thisreaction cell deteriorates to the point where the quality of epitaxialwafers produced using the cell is below an acceptable threshold limit, anew or reconditioned reaction cell assembly is loaded into the CVDsystem and the pre-epitaxy treatment process described above is executedagain.

FIG. 2 is a flow chart illustrating an embodiment for CVD epitaxialgrowth of film on the SiC substrates 200. In step 201 the CVD system isloaded with single-crystal, hexagonal SiC wafers to fill the susceptorwafer pockets. The masks used during the conditioning process of FIG. 1may remain, or fresh masks may be placed on the susceptor to protect anyexposed surface of the susceptor. The system is evacuated, and then aflow of argon and/or hydrogen is established parallel to the wafersurface in step 205 to purge the reaction cell. At step 210 the systemis heated to 1200° C. to 1400° C., and then the argon gas is replacedwith hydrogen gas. Next, at step 215 the temperature is set to a rangeof 1500° C. to 1620° C. and H₂ gas flow is adjusted to achieve aspecific velocity and gradient conditions for epitaxial growth asfollows. A CVD system can be used to perform epitaxial growth 220. Here,the term “epitaxial growth” and the like is understood to include, forexample, executing a manufacturing run to produce an SiC coatedepiwafer, which is to be distinguished from pre-epitaxy steps.

For the process to deposit crystalline SiC films on the crystalline SiCsubstrates, it is found that the conditions must be established toachieve the proper temperature distribution and gas velocity that willresult in the optimal distribution of gas reactants to result in optimalcontrol of particle/surface defects, film thickness, doping and filmsurface morphology. In one example, since only the bottom of thereaction cell is actively heated, the hydrogen gas flow value willimpact the temperature of the other surfaces of the reaction cell. Thiseffect is a result of the hydrogen flow value and the relatively largethermal conductivity of hydrogen, which will act to cool surfaces incontact with the gas flow. It is found that a gas velocity set in therange from 100 to 250 cm/sec, inclusive, with actual value depending onthe gas flow, outer diameter of the reaction zone, and the area of theexit of the reaction zone, will deliver optimum SiC film properties. Fora warm wall multi-wafer CVD system configured as in the examples of Burkor Hecht, if the system is designed to process a configuration ofsimultaneous processing of five substrates, each having a diameter of 76mm, the optimum gas velocity is in the range of 120 to 250 cm/sec, and,in some embodiments, 120 to 160 cm/sec. If the system is made larger forsimultaneous processing of ten to twelve substrates with a diameterranging from 100 to 200 mm, and, in some embodiments, with a diameter of100 mm or six substrates with a diameter of 150 mm, the optimum gasvelocity is in the range of 175 to 250 cm/sec. When this gas flowcondition is set, it is found that it will correspond to a verticaltemperature gradient, in the location between the wafer surface and thetop of the reaction cell above the wafer in the range of 25° C./cm to80° C./cm, with the top of the reaction cell lower in temperature thanthe wafer surface. This corresponds to a range of process controltemperatures of 1500° C. to 1620° C. and hydrogen gas flow in the range65 to 130 slpm (Standard Liters Per Minute). It is this combination ofH₂ flow and gas velocity at a given process temperature that provideconditions that are optimum for controlling SiC film formation. Sincethe composition of the gases used in SiC epitaxy is greater than 99%hydrogen, the gas flow of hydrogen primarily sets the velocity andtemperature gradient conditions. When reactive gasses are added and aCVD process is executed, the SiC films formed under these conditionshave optimal epitaxy film properties and free of particle formation thatresult from ancillary deposits.

Using the process conditions of this method, it is found that thepreferred temperature gradients are significantly smaller than thatreported for SiC growth in hot wall and cold wall reactors (B. Thomaset. al., Materials Science Forum, 457-460, 181, 2004). The properties ofthe epitaxial films produced by the optimized method of this work areimprovements over the results reported in the paper by Thomas, et al.

Referring back to FIG. 2, after the preconditioning process of FIG. 1 iscompleted, the process of FIG. 2 is initiated where in step 201 the CVDsystem can be opened and reloaded with new SiC substrates for growth totargeted film parameters. Typically these substrates are polished andcleaned 4H-SiC substrates tilted away from the c-axis to the<11-20>direction between 2 to 8 degrees. The substrates used aretypically doped with nitrogen to a concentration larger than 1×10¹⁸/cm³.The substrates can be used in such a way that the epitaxial layer isformed on either the silicon face (0001 direction) or the carbon face(000-1 direction). Once the system is loaded with polished substrates,in step 205 the reaction cell is evacuated, and then a flow of argon isestablished parallel to the wafer surface. The system is heated to atemperature in the range 1200° C. to 1400° C., and then the argon gas isreplaced with hydrogen gas (step 210). Next, the temperature is set to arange of from 1500° C. to 1620° C. and H₂ gas flow is adjusted toachieve the velocity and gradient conditions described above for the CVDgrowth (step 201). Prior to the growth of the SiC epitaxy layer thesubstrates can be exposed to a gas mixture which lightly etches thesurface to eliminate any traces of surface damage or contamination fromthe substrate polishing process (optional step 215). At the time forfilm growth, gaseous precursors for Si and C are added to the total gasflow to form a SiC film. The concentration of reactive Si, C andimpurity gases is typically less than 1.5% of the total flow. Duringformation of the epitaxial film, nitrogen, phosphine, diborane ortrimethylaluminum can be added to the gas flow to establish appropriatelevels of donor or acceptor impurities in the film, which will establishthe desired resistivity. Films are typically deposited at pressure offrom 100 to 150 mbar, inclusive. Film thickness can typically range from5 to 150 μm, inclusive, and, in some embodiments, 3 to 120 μm. Typicaldonor/acceptor atom concentrations can range from 1×10¹⁴ to 2×10¹⁹/cm³,inclusive.

Film thickness is typically tested using infrared spectroscopy, whiledonor or acceptor concentration is measured by capacitance voltagetesting. Typically the wafer is tested with a map comprised of asymmetric radius-theta pattern where measurements are made at 2 to 3radius values to a value as large as the wafer radius minus 3 mm, andrepeated 4 to 8 times over rotation values of 360/(number of repeatpoints). For example, a point repeated at a given radius is measured attheta values of 0, 90, 180 and 270 degrees. When the proper gas velocityand temperature gradient are established, it is found that the filmswill exhibit within wafer thickness described by the relation (Maxvalue−min value)/min value ranging from 2 to 12%, inclusive, and thefilms will exhibit within wafer dopant concentration variationsdescribed by the relation (Max value-min value)/min value from 5 to 40%,inclusive. It is found that larger values of the ranges reported abovewill be observed when the largest radius measurement point is takenwithin 8 mm or less of the edge of the substrate.

Under the optimum conditions of gas velocity and temperature gradient itis found that the surfaces of the epitaxial film will be smooth andsurface defects will be minimized. When the roughness of a film ismeasured by atomic force microscopy at scan size of 20×20 μm or less,this will result in an RMS roughness value from 0.2 to 1.2 nm, and, insome embodiments, 0.2 to 1.0 nm, inclusive. At this level of roughnessthe wafers appear generally free of step bunching. The surface defectsare measured using laser light scattering spectrometry. The entire waferis scanned to within 1 to 3 mm of the wafer edge, and then the scannedarea is segregated into 2×2 mm sites. The total surface defects aredetermined by counting the sites with and without defects and thencalculating the ratio of the defect sites to the defect free sites todetermine the fraction of sites free of defects. Then using a Poissondistribution the defect density is calculated from the defect freefraction and the site area. The resulting density of defects ranges from0.25 to 2.0 defects/cm².

EXAMPLE 1

A warm wall CVD system capable of processing 5 pcs of 76 mm diametersubstrates was used for epitaxial growth.

The substrates used were 4H-SiC polytype, tilted 4 degrees away from thec-axis to the <11-20>direction. The substrates had resistivity in therange 0.015 to 0.030 ohm-cm.

A new set of graphite consumables was loaded, baked and coated with aSiC layer as described. The substrates were loaded and processed. Theprocess details and the results measured on a wafer from the processare:

-   -   Run ID/Wafer ID: 1241_AV1006-09    -   Growth temperature: 1585° C.    -   Pressure: 124 mbar    -   Total Hydrogen flow: 72.4 slpm    -   Film Thickness: 5.53 μm, within wafer range 8.1%    -   Growth temperature: 1585° C.    -   Pressure: 124 mbar    -   Total H₂ flow: 72.4 slpm    -   Doping: 5.5×10¹⁵/cm³, within wafer range 15%.    -   Defect density: 0.4 cm⁻²    -   RMS roughness 0.61 nm

EXAMPLE 2

A warm wall CVD system capable of processing 10 pcs of 100 mm diametersubstrates was used for epitaxial growth.

The substrates used were 4H-SiC polytype, tilted 4 degrees to the<11-20>direction. The substrates had resistivity in the range 0.015 to0.030 ohm-cm.

A new set of graphite consumables was loaded, baked and coated with aSiC layer as described. The substrates were loaded and processed. Theprocess details and the results measured on a wafer from the processare:

-   -   Run ID/Wafer ID: A0971_AN2152-16    -   Growth temperature: 1530° C.    -   Pressure: 200 mbar    -   Total H₂ flow: 126 slpm    -   Film Thickness: 7 μm, within wafer range 6.4%    -   Film Doping: 5.7×10¹⁵/cm³, within wafer range 15.7%    -   Defect density: 0.83 cm⁻²    -   RMS roughness 0.30 nm

FIGS. 3, 4 and 5 depict an example of a method 300 of forming anepitaxial SiC film on a plurality of SiC substrates. The SiC substratescan be positioned on a susceptor in a warm wall CVD system, the warmwall CVD system having a reaction cell comprising a susceptor positionedat a bottom, a sidewall, and a ceiling. The susceptor can be activelyheated, and the ceiling and sidewall are not actively heated, but areallowed to be indirectly heated by the susceptor. The method 300 cancomprise inserting an unused reaction cell assembly into a CVD epitaxysystem 310; a first process for treatment of the reaction cellpreparation 340 (FIG. 4); and at least one second process for epitaxialfilm growth 370 (FIG. 5). Here, the term “epitaxial film growth” and thelike is understood to include, for example, executing a manufacturingrun to produce an SiC coated epiwafer, which is to be distinguished frompre-epitaxy steps.

The first process for treatment of the reaction cell preparation 340 cancomprise steps of loading the susceptor with sacrificial substrates 345;sealing and evacuating the reaction cell 350; purging the reaction cellusing argon and hydrogen gases 355; baking the reaction cell in amixture of hydrogen and hydrocarbon gas 360; and performing a CVDdeposition process so as to deposit SiC film on the sidewall and ceilingof the reaction cell 365.

The at least one second process for epitaxial growth 370 can comprisethe steps of allowing the reaction cell to cool 373; placing a pluralityof SiC substrates on the susceptor 376; evacuating and then purging thereaction cell with argon gas 379; heating the susceptor to a temperatureof 1200° C. to 1400° C., inclusive 382; terminating the argon flow andinitiating hydrogen gas flow into the reaction cell 385; flowingparallel to the surface of the wafers a gas mixture of hydrogen, siliconand carbon gases, at total gas velocity of 120 to 250 cm/sec 388;maintaining the temperature of the susceptor at 1500° C. to 1620° C.391; and controlling pressure inside the reaction cell at from 100 to150 mbar, inclusive to achieve a total deposit of 3 to 120 μm of film onthe substrates 394.

The method 300 can be used to form an apparatus 600 (FIG. 6) comprisinga single-crystal hexagonal SiC substrate 610 with at least one epitaxiallayer 620 produced by the method 300. Alternately, more than oneepitaxial layer 620, 630, etc. can be formed on the substrate 610. Inthe apparatus 600, the within wafer total thickness variation can befrom 2 to 12%, inclusive; the within wafer dopant concentration of eachlayer can be from 5 to 40%, inclusive; the top surface of the film canhave an RMS roughness value of 0.2 to 1.2 nm; inclusive; and the densityof surface defects on the film can be from 0.25 to 2.0/cm², inclusive.

It should be understood that processes and techniques described hereinare not inherently related to any particular apparatus and may beimplemented by any suitable combination of components. Further, varioustypes of general purpose devices may be used in accordance with theteachings described herein. The present invention has been described inrelation to particular examples, which are intended in all respects tobe illustrative rather than restrictive. Those skilled in the art willappreciate that many different combinations will be suitable forpracticing the present invention, including the extension of the methodto larger CVD systems accommodating multiple substrates withdiameter >=150 mm.

Although some of various drawings illustrate a number of logical stagesin a particular order, stages which are not order dependent can bereordered and other stages can be combined or broken out. Alternativeorderings and groupings, whether described above or not, can beappropriate or obvious to those of ordinary skill in the art.

Moreover, other implementations of the invention will be apparent tothose skilled in the art from consideration of the specification andpractice of the invention disclosed herein. Various aspects and/orcomponents of the described embodiments may be used singly or in anycombination. It is intended that the specification and examples beconsidered as exemplary only, with a true scope and spirit of theinvention being indicated by the following claims.

We claim:
 1. A method of manufacturing a 4H-SiC epiwafer comprising anepitaxial SiC film on a single-crystal 4H-SiC substrate, the methodcomprising: a. loading the single-crystal 4H-SiC substrate onto asusceptor in a reaction cell of a warm wall CVD system; b. heating thesystem by controlling a temperature of the susceptor in the reactioncell to a range from 1500° C. to 1620° C.; and c. executing amanufacturing run to produce the 4H-SiC epiwafer, the manufacturing runcomprising supplying a gas flow parallel to a surface of thesingle-crystal 4H-SiC substrate, such that a total gas velocity is in arange from 120 to 250 cm/sec and controlling a pressure inside thereaction cell to a range from 100 to 150 mbar, wherein the gas flowcomprises a mixture of hydrogen gas, silicon gas, and carbon gas, inorder to produce the epitaxial SiC film on the single-crystal 4H-SiCsubstrate.
 2. The method of claim 1 further comprising establishing atemperature gradient from a surface of the epiwafer to a ceiling of thereaction cell in a range from 25° C./cm to 80° C./cm.
 3. The method ofclaim 1 further comprising: a. measuring performance metrics of the4H-SiC epiwafer; and b. removing the used reaction cell when themeasured performance metrics of the 4H-SiC epiwafer fall belowacceptable threshold limits.
 4. The method of claim 1, wherein step 1(a)is preceded by pre-treating the reaction cell, the pre-treatingcomprising: a. loading a sacrificial substrate onto the susceptor in thereaction cell; b. sealing and evacuating the reaction cell; c. purgingthe reaction cell using inert and hydrogen gases; d. baking the reactioncell at a temperature in the range 1400° C. to 1700° C. while flowinghydrogen gas mixed with 1% to 10% hydrocarbon gas; e. performing a CVDdeposition process so as to deposit an SiC film on the sidewall andceiling of the reaction cell; and f. removing the sacrificial substratefrom the reaction cell.
 5. The method of claim 1, wherein step 1(c)comprises: a. placing the single-crystal 4H-SiC substrate on thesusceptor in the reaction cell of the warm wall CVD system; b.evacuating the reaction cell and then purging the reaction cell withargon; c. terminating the argon flow and initiating hydrogen gas flowinto the reaction cell; d. establishing the temperature gradient fromthe surface of the epiwafer to the ceiling in the range from 25° C./cmto 80° C./cm; e. flowing parallel to the surface of the epiwafer a gasmixture of hydrogen, silicon and carbon gases, at a total gas velocityof 120 to 250 cm/sec; f. maintaining the process conditions to achieve atotal deposit of from 3 to 120 μm of film on the single-crystal 4H-SiCsubstrate in order to produce the epitaxial SiC film on thesingle-crystal 4H-SiC substrate; g. cooling the system to a temperatureless than 300° C.; and h. removing the single-crystal 4H-SiC substrate.6. The method of claim 5, wherein, in step 5(f), the gas flow furthercomprises doping gas.
 7. The method of claim 5, further comprisingflowing an etching gas into the reaction cell to etch the single-crystal4H-SiC substrate prior to step 5(f).
 8. The method of claim 7, whereinthe etching gas comprises a halogen gas and hydrogen.
 9. The method ofclaim 1, wherein the single-crystal 4H-SiC substrate comprises apolished 4H-SiC wafer with a diameter ranging from 100 to 200 mm andhaving a nitrogen concentration of at least 1×10¹⁸/cm³.
 10. The methodof claim 1, wherein the epitaxial SiC film is deposited on an exposedsilicon surface of the single-crystal 4H-SiC substrate.
 11. The methodof claim 1, wherein the epitaxial SiC film is deposited on an exposedcarbon surface of the single-crystal 4H-SiC substrate.
 12. The method ofclaim 1, wherein in step 1(a), a plurality of single-crystal 4H-SiCsubstrates are placed in the reaction cell.
 13. The method of claim 1,wherein the reaction cell comprises a graphite reaction cell, andfurther comprises coating the reaction cell's graphite components withpyrocarbon or tantalum carbide films prior to assembling the cell foruse in CVD epitaxy.
 14. A single-crystal 4H-SiC substrate with epitaxialSiC film produced by the method of claim 1 wherein, a. a within wafertotal thickness variation of the SiC epitaxial film is from 2 to 12%,inclusive; b. a within wafer dopant concentration of the epitaxial SiCfilm is from 5 to 40%, inclusive; c. a top surface of the epitaxial SiCfilm has an RMS roughness value of 0.2 to 0.2 nm; inclusive; and d. adensity of surface defects on the epitaxial SiC film is from 0.25 to2.0/cm², inclusive.
 15. A method of forming a 4H-SiC epiwafer comprisingan epitaxial SiC film on a single-crystal 4H-SiC substrate positioned ona susceptor in a warm wall CVD system, the warm wall CVD system having areaction cell comprising the susceptor positioned at a bottom, asidewall, and a ceiling, wherein the susceptor is actively heated, andthe ceiling and sidewall are not actively heated, but are allowed to beindirectly heated by the susceptor, the method comprising: a. insertingan unused reaction cell assembly into a CVD epitaxy system; b. a firstprocess for treatment of the reaction cell preparation; and c. a secondprocess for epitaxial film growth; wherein: the first process fortreatment of the reaction cell preparation comprises the steps: i.loading the susceptor with a sacrificial substrate; ii. sealing andevacuating the reaction cell; iii. purging the reaction cell using argonand hydrogen gases; iv. baking the reaction cell in a mixture ofhydrogen and hydrocarbon gas; and v. performing a CVD deposition processso as to deposit an SiC film on the sidewall and ceiling of the reactioncell; and the second process for epitaxial film growth comprises thesteps: vi. allowing the reaction cell to cool; vii. placing thesingle-crystal 4H-SiC substrate on the susceptor; viii. evacuating andthen purging the reaction cell with argon gas; ix. heating the susceptorto a temperature of 1200° C. to 1400° C., inclusive; x. terminating theargon flow and initiating hydrogen gas flow into the reaction cell; xi.flowing parallel to the surface of the wafers a gas mixture of hydrogen,silicon and carbon gases, at a total gas velocity of 120 to 250 cm/sec;xii. maintaining a temperature of the susceptor at 1500° C. to 1620° C.;and xiii. controlling a pressure inside the reaction cell at from 100 to150 mbar, inclusive, to achieve a total deposit of 3 to 120 μm of filmon the substrate.
 16. The method of claim 15, wherein the step of bakingthe unused reaction cell comprises heating the reaction cell to atemperature of from 1400° C. to 1700° C. and maintaining the temperaturefor from 4 to 24 hours.
 17. The method of claim 15, wherein a ratio ofvolumetric flow of carbon to silicon gases is less than 1, but greaterthan 0.05.
 18. The method of claim 15, wherein the temperature of thesusceptor, the total gas velocity, and a throttle setting of thereaction cell are controlled so as to maintain a temperature gradientfrom the substrate to the ceiling in a range from 25° C./cm to 80°C./cm.
 19. The method of claim 15, wherein the total gas velocity ismaintained at 120 to 160 cm/sec, inclusive.
 20. The method of claim 15,wherein the total gas velocity is maintained at 175 to 250 cm/sec,inclusive.
 21. The method of claim 15, wherein a plurality ofsingle-crystal 4H-SiC substrates are placed in the reaction cell.